feat: add back `concat` and `flatten`

[favonia]

Close #18. My hesitation is whether the complex code to make it tail-recursive is worth it:

let concat bs =
  let[@tail_mod_cons] rec go bs =
    function
    | Emp ->
      begin
        match bs with
        | Emp -> Emp
        | Snoc (bs, b) -> (go[@tailcall]) bs b
      end
    | Snoc (xs, x) ->
      Snoc ((go[@tailcall]) bs xs, x)
  in
  go bs Emp

[favonia]

Actually, I might found a possibly better way to write my code without using concat. Let me close this PR for now.